1. Field of the Invention
The present invention relates to a flyback system power source apparatus including a synchronously rectifying element on the secondary side of a transformer.
2. Related Art
As shown in FIG. 4, there is generally a flyback system power source apparatus 50 provided with a synchronously rectifying element SW52 on the secondary side of a transformer T51. In the flyback system power source apparatus 50, if the synchronously rectifying element SW52 on the secondary side is turned on in an on-period of a switching element SW51 on the primary side, then output terminals are short-circuited to each other through a secondary winding N52 and the synchronously rectifying element SW52. Consequently, it is necessary to control the synchronously rectifying element SW52 lest the switching element SW51 on the primary side and the synchronously rectifying element SW52 should be simultaneously turned on. Moreover, there is also the situation such that it is impossible to supply the control signal on the primary side directly to the secondary side in order to insulate the primary side of the transformer T51 from the secondary side thereof.
As a conventional control method of a synchronously rectifying element, there is, for example, the method of detecting the polarity of the current flowing on the secondary side by a current transformer to switch the turning on and off of the synchronously rectifying element on the basis of the detection of the current polarity.
Moreover, as shown in FIG. 5, also the following method was proposed (see, for example, Japanese Patent Application Laid-Open Publication No. Hei 10-74936). The method inserted an inductor L51 in series with the synchronously rectifying element SW52, detected the polarity of the current flowing on the secondary side from the voltage between the both ends of the synchronously rectifying element SW52 and the inductor L51, and turned off the synchronously rectifying element SW52 if the characteristic of a forward current was detected or turned on the synchronously rectifying element SW52 if the characteristic of a reverse direction current was detected.
However, the aforesaid method of using the current transformer has the problem in which the configuration of the circuit for controlling the synchronously rectifying element is complicated and the number of parts is also large to make the circuit mounting area large.
Moreover, also the method of inserting the inductor L51 in series with the synchronously rectifying element SW52 has the problem of the necessity of the addition of the inductor L51 to increase the number of parts.
Moreover, the method of using the parasitic inductance that was caused in the synchronously rectifying element SW52 and wiring in place of the inductor L51 inserted in series with the synchronously rectifying element SW52 to detect the current polarity on the basis of the voltage between both the ends of the inductance component was considered, but the value of the parasitic inductance became very small in this case and consequently it was considered that the detection of the current polarity based on the voltage generated in the parasitic inductance was very difficult.